(1) Field of the Invention
The present invention relates to a signal processing method applied, in a wireless communication system, to signals from a transmitter received by a receiver and, more particularly, to a receiver included in a wireless communication system for mobile communication using an error correction code.
(2) Description of the Related Arts
With mobile communication services offering growing communication speed, wireless communication systems are required to achieve improved frequency efficiency enabling faster wireless communications in a narrower bandwidth.
Using an error correction code is a technique for improving frequency efficiency. Convolution codes and turbo codes are among the error correction codes used for digital data transmissions. An example configuration and an example operation of an existing type of receiver included in a wireless communication system will be described below with reference to FIGS. 8 and 9. FIG. 8 is a block diagram of the configuration of an existing type of receiver. FIG. 9 is a diagram for explaining relationships between received symbols, propagation path estimation results, and soft decision likelihoods associated with an existing type of receiver. In FIG. 8, reference numeral 100 denotes an antenna; 101 a signal receiver; 102 a pilot symbol selector; 103 a propagation path estimator; 104 a propagation path correction part; 105 a demapping part; 106 an error correction decoder; and 200 to 206, 210 and 211 signal lines.
Referring to FIG. 8, a radio signal received at the antenna 100 is inputted to the signal receiver 101. The signal receiver 101 converts the inputted signal into a received baseband signal by processing it at a radio frequency and an intermediate frequency. Also in the signal receiver 101, the received baseband signal is subjected to analog-to-digital conversion. And the digitized received baseband signal is then outputted to the pilot symbol selector 102 via the signal line 200.
The pilot symbol selector 102 separates received pilot symbols and data symbols from the received baseband signal, then outputs the received pilot symbols to the propagation path estimator 103 via the signal line 201 and the received data symbols to the propagation path correction part 104 via the signal line 202.
In the case of the example received baseband signal shown in FIG. 9, out of symbols 0 to 9, the hatched symbols 2 and 7 are received pilot symbols and the other ones are received data symbols. In this case, therefore, the pilot symbol selector 102 outputs, out of the inputted symbols 0 to 9, symbols 2 and 7 to the signal line 201 and symbols 0, 1, 3 to 6, 8, and 9 to the signal line 202.
The propagation path estimator 103 estimates, using the received pilot symbols inputted from the pilot symbol selector 102 and reference pilot symbols predetermined in the system, radio signal propagation paths between a transmitter (not shown) and the receiver. The propagation path HP corresponding to a pilot symbol is estimated, for example, using equation (1) where RP represents the received pilot symbol and AP represents a reference pilot symbol. The HP, RP, and AP are complex numbers. In the example shown in FIG. 9, H2 and H7 represent propagation path estimation results corresponding to pilot symbols 2 and 7, respectively.
                              H          P                =                              R            P                                A            P                                              (        1        )            
Subsequently, the propagation path estimator 103 estimates a propagation path HD corresponding to each data symbol based on the propagation path HP corresponding to each pilot symbol estimated using equation (1). In the example shown in FIG. 9, H0, H1, H3 to H6, H8, and H9 represent propagation path estimation results corresponding to the received data symbols 0, 1, 3 to 6, 8, and 9, respectively. The propagation path estimation results H0, H1, H3 to H6, H8, and H9 corresponding to the received data symbols are obtained by applying linear interpolation, for example, as expressed in equation (2).
                              H          Dn                =                                                                              H                  7                                -                                  H                  2                                                            7                -                2                                      ⁢                          (                              n                -                2                            )                                +                                    H              2                        ⁢                                                  (                                          n                =                0                            ,                              1                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                9                                      )                                              (        2        )            
The propagation path estimator 103 outputs the propagation path estimation results obtained by the above method to the propagation path correction part 104 via the signal line 203.
The propagation path correction part 104 corrects, using the propagation path estimation results inputted from the propagation path estimator 103, effects of propagation paths which have been inflicted on the received data symbols inputted from the pilot symbol selector 102. To do so, the propagation path correction part 104 uses, for example, equation (3).
                              E          d                =                              R            d                                H            d                                              (        3        )            
In equation (3), Rd is a received data symbol, Hd is a propagation path estimation result corresponding to the received data symbol Rd, and Ed is a received data symbol with the effects thereon of the propagation path corrected. The propagation path correction part 104 outputs the data symbol Ed with the effects thereon of the propagation path corrected to the demapping part 105 via the signal line 204.
The demapping part 105 converts, in accordance with a predetermined mapping rule, the received data symbol Ed inputted from the propagation path correction part 104 into a soft decision bit likelihood. An example method of converting a received data symbol into a soft decision bit likelihood will be explained with reference to FIG. 10. FIG. 10 is a diagram for explaining a method of converting a received data symbol into a soft decision bit likelihood using Binary Phase Shift Keying (BPSK) as a mapping rule. It shows a relationship between a reference signal point and a received signal point.
Referring to FIG. 10, symbol points 300 and 301 positioned at I-axis coordinate −A and +A, respectively, are used as reference symbol points in converting symbol data into a soft decision bit likelihood. It is assumed that symbol points 300 and 301 correspond to bit values 1 and 0, respectively. Received data symbol point 302 represents a received data symbol inputted from the propagation path correction part 104 to the demapping part 105. It is positioned at I-axis coordinate X. The demapping part 105 converts the received data symbol 302 into soft decision bit likelihood L using equation (4) where σ2 represents a noise power of additive white Gaussian noise assumed for this case.
                    L        =                              4            ⁢                                                  ⁢            AX                                2            ⁢                                                  ⁢                          σ              2                                                          (        4        )            
Soft decision bit likelihood outputs L0, L1, L3 to L6, L8, and L9 shown in FIG. 9 correspond to received data symbols 0, 1, 3 to 6, 8, and 9, respectively.
The operation of the demapping part 105 has been explained above based on an example case in which BPSK is applied as a mapping rule. The demapping part 105 can operate in a similar manner also in cases where Quadrature Phase Shift Keying (QPSK) or 16 Quadrature Amplitude Modulation (16QAM) is applied as a mapping rule for multilevel modulation. In such cases, the method disclosed in JP-A No. 2004-032125 may be used.
In cases where QPSK or 16QAM is applied as a mapping rule for multilevel modulation, each soft decision bit likelihood output is assumed to include two or more bit likelihoods. When, for example, QPSK is applied as a mapping rule, each soft decision bit likelihood output Ln includes two likelihood values Ln0 and Ln1.
The demapping part 105 outputs the soft decision bit likelihoods calculated as described above to the error correction decoder 106 via the signal line 205.
The error correction decoder 106 performs error correction decoding in a predetermined way using soft decision bit likelihoods L0, L1, L3 to L6, L8, and L9 inputted from the demapping part 105. For example, turbo decoding is performed for error correction decoding. The error correction decoder 106 outputs a decoded bit string obtained as a result of error correction decoding via the signal line 206.
The existing type of receiver shown in FIG. 8 can obtain a decoded bit string by processing a received signal for error correction decoding as described above.
A problem which the present invention aims at solving will be explained with reference to FIGS. 9 and 11. FIG. 11 is a diagram showing relationships between true and estimated propagation-path values on an I-Q plane.
In FIG. 11, the propagation path fluctuation with time is represented by a curve 400 plotted on a complex I-Q plane. Points 401 and 402 also plotted on the complex I-Q plane represent the propagation path estimation results H2 and H7 obtained using the received pilot symbols 2 and 7 shown in FIG. 9. Points 403 and 404 also potted on the complex I-Q plane represent true propagation-path values corresponding to the received data symbols 3 and 4 shown in FIG. 9, respectively.
Assume a case in which propagation paths corresponding to the received data symbols 3 and 4 are estimated using equation (2) for linear interpolation. Propagation path estimation results H3 and H4 corresponding to the received data symbols 3 and 4 shown in FIG. 9 can be represented, in FIG. 11, by points 406 and 407 on a straight line 405 connecting points 401 and 402 on the I-Q plane.
Next, consider the received data symbol 3 shown in FIG. 9. The error included in the propagation path estimation result corresponding to the received data symbol 3 is represented by an error vector 410 between point 403 representing the true propagation-path value and point 406 representing a propagation path estimation result. Similarly, the error included in the propagation path estimation result corresponding to the received data symbol 4 is represented by an error vector 411 between points 404 and 407.
The error vector 411 is longer than the error vector 410. Namely, the error included in the propagation path estimation result represented by point 407 is larger than that included in the propagation path estimation result represented by point 406.
Generally, a propagation path changes continuously as represented by the curve 400 shown in FIG. 11. Therefore, when propagation paths corresponding to data symbols are estimated using propagation path estimation results obtained using received pilot symbols, errors included in propagation path estimation results corresponding to data symbols closer to pilot symbols are considered smaller than errors included in propagation path estimation results corresponding to data symbols farther from pilot symbols.
As stated above in describing the operation of an existing type of receiver, soft decision bit likelihoods used for error correction decoding are calculated using propagation path estimation results. Namely, the magnitudes of errors included in propagation path estimation results correspond to the magnitudes of errors included in soft decision bit likelihoods.
Hence, errors included in soft decision bit likelihoods corresponding to data symbols closer to pilot symbols are considered smaller than errors included in soft decision bit likelihoods corresponding to data symbols farther from pilot symbols.
When signals are processed in existing types of receivers and transmitters, however, all soft decision bit likelihoods are treated equally without any consideration given to the magnitudes of errors included in them. In error correction decoding, therefore, the reliability of error correction is considered unfavorably affected by soft decision bit likelihoods including large errors. This results in degrading the system performance, lowering the communication quality and frequency efficiency of the system.
The above description of related arts has covered a problem of propagation path fluctuation with time. A similar problem is caused also by propagation path fluctuation with frequency attributable to the frequency selective for multipaths.
A method of solving such problems is disclosed in JP-A No. 2007-135021. In the method, an important signal for error correction decoding is provided for a symbol positioned closely to a pilot symbol so as to improve the communication quality. The method, however, relates to transmission processing according to a communication method, so that it is not easily applicable to a wireless communication system using an already established communication method.